1. Field of the Invention
The present invention relates generally to a tunable optical filter applicable to a system such as an optical fiber communication system, and more particularly to a tunable optical filter in which the shape of a characteristic curve giving a wavelength characteristic of transmittance changes along a transmittance axis.
2. Description of the Related Art
In recent years, a manufacturing technique and using technique for a low-loss (e.g., 0.2 dB/km) optical fiber have been established, and an optical fiber communication system using the optical fiber as a transmission line has been put to practical use. Further, to compensate for losses in the optical fiber and thereby allow long-haul transmission, the use of an optical amplifier for amplifying signal light has been proposed or put to practical use.
An optical amplifier known in the art includes an optical amplifying medium to which signal light to be amplified is supplied and means for pumping the optical amplifying medium so that the optical amplifying medium provides a gain band including the wavelength of the signal light. For example, an erbium doped fiber amplifier (EDFA) includes an erbium doped fiber (EDF) as the optical amplifying medium and a pumping light source for supplying pump light having a predetermined wavelength to the EDF. By preliminarily setting the wavelength of the pump light within a 0.98 xcexcm band or a 1.48 xcexcm band, a gain band including a wavelength of 1.55 xcexcm can be obtained. Further, another type optical amplifier having a semiconductor chip as the optical amplifying medium is also known. In this case, the pumping is performed by injecting an electric current into the semiconductor chip.
As a technique for increasing a transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. The plural optical carriers are individually modulated to thereby obtain a plurality of optical signals, which are wavelength division multiplexed by an optical multiplexer to obtain WDM signal light, which is output to an optical fiber transmission line. On the receiving side, the WDM signal light received is separated into individual optical signals by an optical demultiplexer, and transmitted data is reproduced according to each optical signal. Accordingly, by applying WDM, the transmission capacity in a single optical fiber can be increased according to the number of WDM channels.
In the case of incorporating an optical amplifier into a system adopting WDM, a transmission distance is limited by a gain characteristic (wavelength characteristic of gain) which is often called as a gain tilt. For example, in an EDFA, a gain deviation is produced at wavelengths in the vicinity of 1.55 xcexcm. When a plurality of EDFAs are cascaded to cause accumulation of gain tilts, an optical SNR (signal-to-noise ratio) in a channel included in a band giving a small gain is degraded.
To cope with the gain tilt of an optical amplifier, a gain equalizer may be used. Before a degradation in optical SNR in a certain channel becomes excessive due to accumulation of gain tilts, gain equalization is performed by the gain equalizer provided at a suitable position.
A tunable optical filter is known as an optical device usable as the gain equalizer. In the tunable optical filter, a wavelength characteristic of transmittance (or loss) (wavelength dependence of transmittance) is variable. For example, the wavelength characteristic of the tunable optical filter is set or controlled so as to cancel the gain tilt of an optical amplifier, thereby reducing an interchannel deviation of powers of optical signals at the receiving end.
Conventionally known is a tunable optical filter having a mechanically movable part. In this kind of tunable optical filter, for example, an angle of incidence of a light beam on an optical interference film or a diffraction grating is mechanically changed, thereby changing a center wavelength in a transmission band or a center wavelength in a rejection band. That is, the shape of a characteristic curve giving a wavelength characteristic of transmittance changes along a wavelength axis. Further, a tunable optical filter provided by Photonics Technologies, Inc. applies a split-beam Fourier filter as the basic principles to make variable not only the center wavelength, but a rejection quantity (transmittance) itself by mechanical means. That is, the shape of a characteristic curve giving a wavelength characteristic of transmittance is variable not only along the wavelength axis, but along a transmittance axis.
Further, as a tunable optical filter capable of changing a wavelength characteristic of loss by electrical means without using any mechanically movable part, a waveguide type Mach-Zehnder (MZ) optical filter and an acousto-optic tunable filter (AOTF) are known, for example.
Further, an optical bandpass filter capable of varying a center wavelength applying a birefringent filter as the basic principles has been proposed.(Japanese Patent Laid-open Publication No. 6-130339).
The tunable optical filter having a mechanically movable part has defects such that high-speed operation is difficult and reliability is lacking. Further, the MZ optical filter and the AOTF at present have defects such that (1) a drive voltage is high, (2) a power consumption is large, (3) a temperature stabilizing device is required to cause an unavoidable enlargement of scale, and (4) reliability cannot be obtained.
It is therefore desired to design a tunable optical filter that can satisfy such conditions that (1) no mechanically movable part is included to obtain high reliability, (2) the filter is controllable by electrical means, and (3) a drive voltage is low and a power consumption is small.
As a candidate for the tunable optical filter satisfying these conditions, a tunable optical filter described in Japanese Patent Laid-open Publication No. 6-130339 is noticeable. This tunable optical filter has a variable Faraday rotator for giving a variable Faraday rotation angle, in which the shape of a characteristic curve giving a wavelength characteristic of transmittance is changed along the wavelength axis according to a change in the Faraday rotation angle. However, the shape of the characteristic curve cannot be changed along the transmittance axis. In the prior applications of the gain equalizer,for example, it is required that a loss depth in a rejection band is variable, it therefore cannot be said that this tunable optical filter always have a sufficient performance as a gain equalizer.
It is therefore an object of the present invention to provide a tunable optical filter in which the shape of a characteristic curve giving a wavelength characteristic of transmittance changes along the transmittance axis. The other objects of the present invention will become apparent from the following description.
In accordance with an aspect of the present invention, there is provided a tunable optical filter comprising first and second polarizers, a birefringent element, and a Faraday rotator. Each of the first and second polarizers has a transmission axis determining a polarization axis of transmitted polarized light. The birefringent element is provided between the first and second polarizers to give a phase difference between two orthogonal components of transmitted polarized light. The phase difference is determined by an optic axis of the birefringent element. The Faraday rotator is provided between the first and second polarizers to give a variable Faraday rotation angle to transmitted polarized light. The order of arrangement of the birefringent element and the Faraday rotator and the relative positional relation between the optic axis of the birefringent element and the transmission axis of each of the first and second polarizers are, for example, set so that the shape of a characteristic curve giving a wavelength characteristic of transmittance changes along a transmittance axis according to a change in the Faraday rotation angle.
With this configuration, the order of arrangement and the relative positional relation are set in a specific manner, so that the shape of the characteristic curve is variable along the transmittance axis, thus, a loss depth in a rejection band can be changed, thereby achieving one of the objects of the present invention.
In the present specification, the term of xe2x80x9ctransmittancexe2x80x9d is defined as a power transmittance.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.